1. Field of the Invention
The present invention relates to an automatic transmission for use in an automotive vehicle, and more particularly to an automatic transmission having an overdrive gear mechanism positioned between a hydrokinetic torque converter and an underdrive mechanism.
2. Description of the Prior Art
As is well known, an automatic transmission for an automotive vehicle operates to automatically change gear reduction ratios in response to vehicle speed and load on the engine. An automatic transmission consists in general of a torque converter and an underdrive gear mechanism which is coupled to the converter and establishes two or more gear reduction ratios of no less than 1. Recently, a demand for an automatic transmission with an overdrive has arisen from the viewpoint of fuel economy, and emission control of the engine.
If an overdrive gear mechanism is added to an automatic transmission having a torque converter and an underdrive gear mechanism, when gear strength and capacity of frictional engaging means in the overdrive gear mechanism are desired to be decreased for improving durability and manufacturing cost, it is advantageous to position the overdrive gear mechanism between the torque converter and the underdrive gear mechanism rather than on an output side of the underdrive gear mechanism. (rear-side type). In other words, the output side of the overdrive gear mechanism is connected to an input side of the underdrive gear mechanism (front side type). In addition, the automatic transmission is equipped with a torque converter and a hydraulic control device, so that an oil pump is required for delivering hydraulic pressure thereto. The oil pump should be driven by being coupled to an pump impeller in the torque converter, so that in the case of the front side system, the oil pump is provided within a wall partitioning the torque converter from the overdrive gear mechanism. Furthermore, the overdrive case which encompasses the overdrive gear mechanism therewith partitions the overdrive gear mechanism from the underdrive gear mechanism, with the result that the underdrive gear mechanism is housed in a chamber or space which is independent of the underdrive gear mechanism.
An automatic transmission of this type suffers from a disadvantage in that air pressure in spaces within the overdrive gear mechanism and underdrive gear mechanism varies or increases thereby causing damage to oil seals provided within the automatic transmission. Additionally oil may be caused to blow out from the automatic transmission through a filler tube, into which an oil level gage is inserted. To cope with this, there is provided a breather circuit communicating with the atmosphere for minimizing the difference between the pressures in these spaces and atmospheric pressure. Oil which has lubricated and cooled the overdrive gear mechanism and underdrive gear mechanism is forced from a rotating body towards a peripheral portion of a casing due to a centrifugal force and then clings to the inner surface of the wall encompassing the overdrive case or transmission case. Accordingly, if the end of a breather circuit is positioned directly in the aforesaid inner surface, then oil tends to close the breather circuit or leaks therein. It follows from this that the breather circuit should run through complex passages formed in a oil pump body and an oil pump cover in the axial direction, rather than in the radial direction in a manner to connect with the overdrive gear mechanism and underdrive gear mechanism. In addition, in case the overdrive case is provided independently of the transmission case, then breather means should be provided separately. This leads to a complicated construction of housings or cases and an increase in manufacturing cost.